Add setting to isolate zoom changes by default.

chrome/browser/resources/options/content_settings.js

Index: chrome/browser/resources/options/content_settings.js
diff --git a/chrome/browser/resources/options/content_settings.js b/chrome/browser/resources/options/content_settings.js
index edcadafe4890d5ff79bbd9d97ac524b253f79b77..63b2897e45508d366b5f390bb803adda8ab91ed3 100644
--- a/chrome/browser/resources/options/content_settings.js
+++ b/chrome/browser/resources/options/content_settings.js
@@ -261,6 +261,16 @@ cr.define('options', function() {
   };
 
   /**
+   * Shows/hides the explanation of per-tab zoom scope under the list of any
+   * existing zoom level exceptions.
+   * TODO Is this how we want to present this to the user?
+   * @param {boolean} show Whether to show the explanation.
+   */
+  ContentSettings.showZoomScopeExplanation = function(show) {
+    $('zoom-levels-ignored-explanation').hidden = !show;
+  };
+
+  /**
    * Set the default microphone device based on the popup selection.
    * @private
    */